Emacs opening post files as latin-1 and the rest as UTF-8

[Ismael Valladolid Torres]

I am running Debian testing with the mutt and emacs21 packages
installed (then they are Mutt 1.5.13 and GNU Emacs 21.4.1).

In my .emacs there's the following:

(set-locale-environment (getenv "LANG"))

Where before I had this which also gave the same problem:

(when (string-match "UTF-8" (getenv "LANG"))
  (set-language-environment "UTF-8"))

What I am trying to achieve is for emacs to open any new file or
existing file editing it in the UTF-8 language environment ("u" must
be showed in the lower left corner of emacs). This works for any file
except when I go to mutt and create a new mail or reply to an existing
one. Then the file "/tmp/mutt-blahblah" is opened for editing in
latin-1 mode ("1" is on my lower left corner then). Which is a mess as
many of them are in spanish and UTF-8 special characters appear
mangled in the post being edited.

What can be making Emacs not to change the language environment when
opening a mutt temporal post file?

Elisp packages installed are:

emacs-goodies-el                                install
post-el                                         install

Any ideas welcome!

Cordially, Ismael